It has already been noted many times that one key difference between Israel and the U.S. over Iran is that Washington can wait far longer than Israel before it decides that it has no choice but to use force in order to destroy the Iranian nuclear program. In the simplest of terms, while the U.S. can keep trying negotiations and sanctions until five minutes before midnight, when Iran crosses the nuclear finishing line, Israel would have to already act at 11:30. In mid-August, the chairman of the Joint Chiefs of Staff, General Martin Dempsey, remarked "we admit that our are clocks ticking at different paces."

There are multiple reasons given why the U.S. can afford to wait. The most commonly discussed explanation is the much greater firepower of the U.S. Air Force in comparison with the Israel Air Force. Presumably, against a fleet of B-2 bombers, there is no "zone of immunity" that Iran can create for its Iranian nuclear facilities. Dempsey gave another explanation, "Israel sees the Iranian threat more seriously than the U.S. sees it, because a nuclear Iran poses a threat to Israel's very existence."

A third reason given by the U.S. for why it can wait has to do with its confidence that it will have the intelligence it needs to detect that Iran has crossed the nuclear threshold. In early March 2012, President Barack Obama told The Atlantic's Jeffrey Goldberg, "Iran does not yet have a nuclear weapon and is not in a position to obtain a nuclear weapon without us having a pretty long lead time in which we will know that they are making that attempt." Perhaps Obama was thinking that as long as Tehran did not kick out the inspectors from the International Atomic Energy Agency and shut down their cameras, as it made a final dash to nuclear weapons in what experts call "nuclear breakout," the U.S. would not have to consider the use of force against Iran.

Although he did not say this explicitly, Obama left open the possibility that in the meantime, Iran could move forward with its program in the coming months, while facing sanctions and diplomatic pressure, as long as it didn't actually cross the nuclear weapons threshold, it would not face an American attack. As noted in this column previously, there is a huge risk with accepting an Iranian threshold strategy, which former Secretary of Defense Robert Gates pointed out in 2010. At that time he said that if Iran reached the nuclear "threshold," but did not assemble the bomb, the U.S. would not know that it had completed the final assembly of an atomic weapon.

In the meantime, Iran has been working to shorten this threshold phase, making the intelligence challenge even greater. By producing a growing stock of 20 percent enriched uranium, it has cut in half the time needed to enrich uranium to the 90% weapons-grade level. In the meantime, by next spring its stock of low enriched uranium will be sufficient for at least eight atomic bombs, upon further enrichment. In July, the head of the Atomic Energy Agency of Iran, Fereydoun Abbas-Divani, boasted that Iran now has the technology to move quickly toward producing weapons-grade uranium.

According to IAEA reports, Iran has been working on warheads outfitted to carry an atomic weapon for the 1300 kilometer range Shahab-3 missile. If all that is left to complete an operational nuclear weapon is a few more weeks of work, then letting Iran reach a threshold capacity is very dangerous for obvious reasons: When nuclear breakout occurs, Iran can quickly build a substantial nuclear arsenal.

But waiting for the very last minute to act against Iran when it actually crosses the nuclear threshold also carries a steep diplomatic price for the United States. Over time, many states, especially in the Persian Gulf, will conclude that the U.S. will never take any action against Iran, even though the Iranian threat is growing. This was illustrated in another interview Goldberg conducted with the UAE ambassador to Washington, Yusuf al-Otaiba, who warned him: "There are many countries in the region, who if they lack the assurance the U.S. is willing to confront Iran, they will start running for cover towards Iran."

What the UAE ambassador was essentially saying was that as time goes on, if there are growing doubts about American resolve to destroy the Iranian nuclear program, and Tehran succeeds in "decoupling" (to use a Cold War term) the Arab states from Washington, then the U.S. alliance structure in the Arabian Peninsula might eventually collapse. Students of international politics probably recall the distinction drawn by US academics, like Kenneth Waltz, between states that seek to unite and "balance" a common threat by creating an alliance and states that give up and get on the "bandwagon" of their adversaries. Accepting Iran with a threshold nuclear capacity will eventually result in Arab states getting on the Iranian bandwagon.

Indeed, senior Arab officials in the Persian Gulf point out that Qatar's alliance with the U.S. began to change after the Bush administration released the 2007 National Intelligence Estimate (NIE). In particular, many were disturbed by the language used in its summary which contended that Iran had halted its nuclear weapons program in 2003. As a result of the NIE, the Qataris immediately began to doubt the resolve of the U.S. to deal with the Iranian challenge. Consequently, Qatar changed its policy toward Washington, and adopted a pro-Iranian orientation, presumably in order to safeguard its security.

Because of the Syrian crisis, it appears that Qatar has shifted back to the Sunni bloc for now. But that tactical change does not eliminate the fact that there is a big risk for the West if it accepts a threshold policy for Iran: what happened with Qatar in 2007 could easily spread to Saudi Arabia and the rest of the Gulf states, which would seek to reduce their ties with Washington over time and acquiesce to Iran's demands for much higher oil prices in OPEC.

For all these reasons, letting Iran reach the status of a nuclear threshold power is a big mistake. In January 2012, Secretary of State Hillary Clinton told Congress that the U.S. objected to Iranian nuclear weapons capabilities and not just to the production of the weapons themselves. But how is the U.S. translating that position into practical policy, especially when it comes to the use of force, when it becomes clear to the White House that diplomacy has reached a dead end? For Iran, Washington's tolerance of a nuclear threshold capacity allows it to build up the size of its future nuclear forces, to split the U.S. from its Arab allies over time, without having to risk an American military strike. If this situation continues, it will become far harder in the future for any state to stop Iran's determination to acquire nuclear weapons.